Detection of water distribution and dynamics in body of winter wheat based on nuclear magnetic resonance

Abstract: The nuclear magnetic resonance (NMR) technique is a noninvasive and nondestructive way of probing water content in plants, and has been broadly used in the studies on plant-water relationship. In order to investigate water distribution in a living winter wheat plant, the T2 relaxation parameters of NMR were used to measure the moisture contents (MC) in leaf, stem, and spike of winter wheat. The results showed that the amplitude of T2 relaxation spectrum was linearly correlated to the mass of water in the plant sample. And according to the T2 relaxation properties of wheat leaf, stem, and spike, the T2 relaxation spectra could be divided into two components. The fresh weight could be estimated through linear regression using the peak areas of each component of T2 spectra. Then, the moisture contents of each organ could be obtained. To verify the moisture content estimation function, the moisture contents of organs of seven wheat varieties were measured with both NMR and traditional oven-drying methods. The root mean square error (RMSE) was adopted to measure the accuracy. The results showed that the RMSE of two kinds of measured moisture contents of leaf, stem and spike were 5.3%, 3.5% and 3.3%, respectively. The NMR detection method had high detection precision for moisture content. By comparison, the detected moisture contents in spike and stem were more precise, followed by leaf measurement. The larger error for wheat leaf was probably due to a relatively lower signal-noise ratio, and a reason for this might be the lower level filling degree relative to stem and spike. Then the method was applied to monitor the long-term and diurnal changes of the moisture contents in living wheat plants at different growth stages. The results showed that moisture content of the second upper leaf decreased from 79% to 54% during grain milk stage and ripening. The moisture content in flag leaf remained relatively stable, which kept at 78% and then decreased to 72% until the grain dough stage. The moisture contents of the first and second stems below spike also decreased from 69% to 60%. The nutriment in leaf and stem was activated and transferred into wheat grain during the stage of grain filling. This process was accompanied by a decrease of the moisture content in spike from 61% to 31%. The moisture contents of the flag leaf and second upper leaf decreased first and then increased during the stages of grain milk and ripening. At grain milk stage, leaves moisture contents were about 77% at 8:00 and reduced gradually to 74% at 14:00 (at 16:00 for flag leaf), then to 76% at 20:00. It means the moistures of leaves could recover to their initial state after decreased at grain milk stage. However, at ripening stage, the moisture content of flag leaf decreased from 70% at 8:00 to minimum (53%) at 16:00, then to 60% at 20:00. The second upper leaf showed the same tendency that the moisture content decreased from 66% to 45% and merely increased to 55% thereafter. Since the method based on NMR can continuously monitor the same wheat plant without destruction and invasion, this study can reveal the water dynamics and aging process of winter wheat more directly and precisely. The results will lay a theoretic foundation for the study on water consumption of winter wheat growing healthy and rational irrigation schedules. During next study, more wheat plants and high frequency detecting should be taken into account to demonstrate the general significance. At the same time, different environment conditions should be settled for a deeper exploration of the water distribution and movement within the wheat body.